Physical ability/fitness is built up through progressive overload, it doesn't just happen. It's a not a light switch, you can't just go from playing 40 games a season to 60 games without having slowly developed a players fitness.
When Arteta was pushing Saka and his teammates in recent seasons, he's building them up to be able to perform the type of gruelling schedules that the best teams have to play, usually on four fronts.
Why do you think so many clubs struggle when they secure top 4 and then subsequently fall out? They go from having to play once a week to every 3 days and the players aren't physically equipped to deal with the load.
We on the other hand reached the Q/F's and sustained a title challenge with a group of players who were being physically prepped in previous systems, in many dead rubber games, so players can deal with the schedule and play every 3 days.
